I doubt you can get an external antenna. The antenna's that are built in are optimized for gsm/gprs. The handheld will only use as much power as it needs to broadcast the signal, and a bigger antenna is not going to help.

Interestingly enough, the reason for the stubby antenna's on the iDEN devices is not for better signal, it's actually to direct the radio waves away from your head because of the power that they have to broadcast at. This is true for all iDEN devices. Makes me want to keep one close to me...